Centering arrangement for cathode ray beams



July 29, 1941.

H. BAHRING 2,250,884

CENTERING ARRANGEMENT FOR CATHODE RAY BEAMS Filed Oct. 22, 1938' INXENTOR. RT 8 HRI NG Patented July 29, 1941 cries CENTERING ARRANGEMENT F03. CATHODE RAY BEAMS Herbert Bahring, Berlin assignor to Fernseh A Zehlcndorf, Germany Application October 22 In Germany Claims.

This invention relates to magnetic systems for deflection of defined cathode ray beams, and particularly to means for establishing a center position about which a defined cathode ray beam is to be deflected back and forth.

In television it is known to reproduce an image by scanning a fasciculated cathode ray beam, modulated by received picture signals, across a fluorescent screen according to a predetermined pattern. Deflection of the cathode ray beam in two directions can be accomplished by means of magnetic fields produced by causing currents of certain wave shapes to flow through a number of deflecting coils, as is well known in the art.

It is, of course, desirable that the reproduced image should take in a field, on the fluorescent screen, which is symmetrical about the center of the cathode ray tube. It is also known in the art that this requirement cannot always be met by the use of the deflecting fields only, and it is common practice to employ additional constant magnetic fields tomove the center lines about which the cathode ray beam is deflected. These additional fields then compensate effects tending to make the undefiected cathode ray beam deviate from the center of the fluorescent screen, such as for instance the earths magnetic field.

Two methods of generating such additional or centering fields are known so far: first, by providing [coils in addition to those used for deflection and by causing an adjustable direct current to flow through these coils; second, by supplying a direct current to the deflecting coils through a shunted path. The disadvantage of the first method is that it largely complicated the mechanical structure of the deflecting system. The disadvantage of the second method is that the resistance of the shunted direct-current supply path must be large compared with that of the deflecting coils in order to avoid distortion of the wave shape of the deflecting current. The consequence thereof is that a comparatively small direct current is gained even at high unidirectional voltages.

It is the object of this invention to overcome these disadvantages by incorporating the centering arrangement in the deflecting circuit. An adjustable and reversible flow of direct current through the deflecting :coils is then obtained.

Advantages of my method are that no additional coils are required for centering and that a large centering current or field is obtained'at low unidirectional voltages, because the resistance of the centering arrangement may be made small.

-Zehlendorf, Germany, ktiengesellchaft, Berlin- 1938, Serial No. 236,413 October 23, 1937 Other objects and advantages of the invention will become evident in the following.

The invention provides a bridge circuit of ohmic resistances arranged in the deflecting system, in a manner enabling to control the flow of direct current through the deflecting coils both in respect to magnitude and direction of current flow. The bridge circuit is shunted by a condenser, which represents substantially a short circuit for the deflecting currents. The capacitive reactance of the condenser is chosen to be small compared with theinductive reactance of the output circuit, in which the deflecting coils are disposed. Furthermore, it is provided that the aforementioned shunting condenser and the ohmic resistance of the bridge circuit have a time constant which is greater, by for instance in the order of five times, than the period of the deflecting current. A condenser of such size prevents the wave shape of the deflecting current, which is usually that of a saw tooth, from suffering distortion, that is, all the sinusoidal components of the deflecting current suffer no attenuation or phase shift due to the centering device.

The drawing shows several embodiments of the invention. Figure 1 shows a bridge circuit using a twin potentiometer; Figure 2 shows a bridge circuit in which a fixed voltage divider is used in one branch and a potentiometer in the other; Figure 3 shows the bridge according to Figure l incorporated in a push-pull circuit; and Figure 4 shows the bridge according to Figure 2 in a single-sided circuit.

The invention may now be described in detail in connection with the drawing. Figure 1 shows a bridge circuit with the potentiometers l and 2, whose sliding contacts are moved at the same rate in opposite sense as indicated by the arrows. A direct-current source is connected between points 3 and d. The deflecting circuit including the deflecting coils (not shown) through which the centering current is to flow is connected to the terminals 5 and 6. A condenser 1 is connected between the terminals 5 and 5 and shunts the bridge. This condenser 1 represents substantially a short circuit for the deflecting current. If the resistances in the branches of the bridge are made large compared with the ohmic resistance of the deflecting coils, it is possible to adjust the bridge in such a manner that almost the total current delivered by the directcurrent source is caused to flow through the defleeting coils. Considering the fact that this current can be reversed the range over which the undefiected cathode nay beam can be shifted is extremely great.

In Figure 2 the bridge consists of the fixed resistors 8 and 9 and the variable potentiometer 2. The source of direct current is again applied between the terminals 3 and 4. The deflecting circuit including the deflecting coils (not shown) through which the centering current is to flow is connected to the terminals 5 and 6 which are again shunted by a condenser 'I. By moving the sliding contact of the potentiometer 2 the magnitude of the centering current can be varied as well as its direction. I prefer to make the resistors 8 and 9 equally large, while the resistance of potentiometer 2 is preferably chosen to be in the order of ten times the value of the resistor 8. If such dimensions are chosen and if the ohmic resistance of the deflecting coils is small compared with that of potentiometer 2, the maximum centering current will be greater than 90% of the total direct current supplied. Taking the possibility of reversing the direction of current flow into consideration, the magnitude of the centering field can be varied over a range corresponding to almost twice the direct current supplied.

In both the above-described circuits it is advisable to make the internal resistance of the direct-current source between terminals 3 and 4 large compared with the lowest resulting resistance of the bridge arrangement. In this case the requirement that the time constant rendered by condenser 'I and the resulting resistance be greater than the period of the deflecting current can readily be met. In practice this requirement is easily fulfilled by disposing the bridge in the path of the direct-current supply for the circuit in which the deflecting current is generated.

Figure 3 shows the application of the invention to a push-pull amplifier circuit. A sawtooth voltage may be applied to the control grids of the tubes I and II. This causes a sawtooth current to flow through the deflecting coils I2 which are connected between the anodes of the tubes In and II. The unidirectional anode current is supplied to the respective tubes through the potentiometers I and 2, the two windings of choke coil I3 and the resistors I4 and I5. The bridge arrangement corresponds to that shown in Figure 1. If now, the sliding contacts of the potentiometers I and 2 are moved in opposite directions as indicated on the drawing by the arrows, more direct current will flow through the resistor I than through the resistor I4. As both tubes are adjusted to draw equal amounts of direct current, a portion of the current flowing through the resistor I5 will flow through the deflecting coils I2 to the anode of tube II. Thus a centering field is established. The reversibility of the centering current obviously follows from the description of Figure 1. It may be understood that the magnitude of the maximum centering current is limited by the unidirectional anode current through the tubes I0 and II, as the bridge is connected to the direct-current supply only through the tubes I0 and II. If the maximum centering current is desired to be greater than the unidirectional anode current, it is necessary to close a switch Ilia, thus connecting the bridge directly to the direct current source through a resistor I6. The maximum centering current is then no longer limited by the unidirectional anode current of the tubes.

In order to avoid distortion of the deflecting current it is advisable to make the time constant of the deflecting coils I2 and the tubes I0 and II equal to that of the resistors I4 and I5 and the choke coil I3. The centering arrangement does not enter into the time constant consideration if the shunting condenser I is large enough to be considered substantially a short circuit for the fundamental of the deflecting current wave. The unidirectional voltage drop required across the potentiometers I and 2 decreases with decreasing ohmic resistance of the deflecting coils I2, of the resistors I4 and I5, and of the resistance of the choke coil I3. The voltage drop lies in the order of between 50 and volts.

The slight unbalance in the anode voltages of the tubes II) and II of a few volts is of no harm to the amplification if pentode tubes are used in the push-pull arrangement.

Figure 4 shows the application of the invention to a single-sided amplifier for .the deflecting current. A voltage of for instance saw-tooth shape may be applied to the control grid of a tube I8. An output transformer II couples the deflecting coils I2 to the anode circuit of the tube I8. Resistors 8 and 9 and a potentiometer 2 constitute a bridge corresponding to that shown and described in Figure 2. Again shunting condenser I is provided as a substantial short circuit for the deflecting current. By moving the sliding contact of the potentiometer 2 the magnitude and. the direction of the centering current can be altered. As described above the maximum centering current is again limited by the unidirectional anode current through the tube I8. If it is desired that the maximum centering current be greater than the unidirectional anode current, it is necessary to close a switch Illa thereby connecting the bridge directly to the direct-current source, which is connected between the terminals 3 and 4, through a resistor I9.

What I claim is:

1. In a scanning system, the combination of a repeater having a control element, means for applying a scanning voltage wave to said control element, a supply circuit for applying unidirectional operating current to said repeater, an output circuit connected to said repeater, a bridge circuit coupled to said supply and output circuits to direct a portion of said operating current through said output circuit, and means for adjusting said bridge circuit to control the flow of said portion of said operating current in said output circuit.

2. In a scanning system, the combination of a repeater having a control element, means for applying a scanning voltage wave to said control element, a supply circuit for applying unidirectional operating current to said repeater, an output circuit connected to said repeater, a bridge circuit coupled to said supply and output circuits to direct a portion of said operating current through said output circuit, and means for adjusting said bridge circuit to control the magnitude and the direction of the flow of said portion of said operating current in said output circuit.

3. A scanning system comprising a repeater having a control element, means for applying a scanning voltage wave to said control element, a bridge circuit having in its branches a plurality of resistors and a potentiometer, a source of unidirectional current, an inductance element connected in series with said repeater and said current source and included in the diagonal of said bridge circuit parallel to said potentiometer, inductive scanning means coupled to said inductance element, a condenser connected in series with said scanning means, and means for connecting said condenser in the other diagonal of said bridge circuit.

4. A scanning system comprising a repeater having a control element, means for applying a. scanning voltage Wave to said control element, a bridge circuit having in its branches a plurality of resistors and a potentiometer, a source of unidirectional current, an inductance element connected in series with said repeater and said current source and included in the diagonal of said bridge circuit parallel to said potentiometer, inductive scanning means coupled to said inductance element, a condenser connected in series with said scanning means, means for connecting said condenser in the other diagonal of said bridge circuit, and a resistor shunting said series combination of said repeater and said inductance element.

5. A scanning system comprising a pair of repeaters connected in push-pull relation, each of said repeaters having a control element and a cathode, means for applying scanning voltage Waves to said control elements, a source of unidirectional operating current, a bridge circuit having potentiometers in its branches, a condenser connected in the diagonal of said bridge circuit between points on said potentiometers, inductive scanning means connected in the common output circuit of said repeaters, a pair of impedances, means including said impedances for connecting said scanning means across said condenser, and means for connecting said current source between said cathodes and the junction point of two ends of said potentiometers.

HERBERT BAHRING. 

